1. Field of the Invention
The present invention relates to light valves, and in particular, to light valves utilizing thin liquid crystal transducer pixel cells having self-aligned pillars for supporting the top glass substrate.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye. Two factors dictate the minimum frequency necessary for switching.
The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick (&gt;1 .mu.m) liquid crystal (LC) transducers. However, thin (&lt;1 .mu.m) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1A shows a top view of a conventional thin LC transducer pixel cell. FIG. 1B shows a cross-sectional view of the thin liquid crystal transducer along line A-A' of FIG. 1A. FIG. 1C shows a cross-sectional view of the thin liquid crystal transducer along line B-B' of FIG. 1A.
Thin LC transducer pixel cell 100 comprises a layer of liquid crystal (LC) material 102 sandwiched between a top plate 104 and a bottom plate 106. Top plate 104 is physically supported and separated from bottom plate 106 by support pillars 105. Top plate 104 is a translucent material, typically glass. Bottom plate 106 is a reflective pixel electrode layer.
Pixel electrode layer 106 actually consists of two layers: a metal electrode layer 108 on top of an electrode liner layer 110. Pixel electrode layer 106 is delineated into individual pixel electrodes 130 by intervening trenches 118.
Pixel electrode layer 106 lies on top of an upper intermetal dielectric layer 112 that is one component of an interconnect scheme. This interconnect overlies a capacitor structure formed within an underlying silicon substrate (not shown). Upper intermetal dielectric layer 112 electrically insulates pixel electrode 130 from lower metallization layer 114. The underlying capacitor structure is in electrical communication with pixel electrode 130 through metal-filled via 116.
FIGS. 2AA-2EC illustrate the conventional process for forming a thin LC transducer pixel cell. For purposes of convention, all FIGS. 2.sub.-- A illustrate a top view of the pixel cell, all FIGS. 2.sub.-- B illustrate a cross-sectional view of the pixel cell along line A-A' of the FIG. 2.sub.-- A, and all FIGS. 2.sub.-- C illustrate a cross-sectional view of the pixel cell along line B-B' of the FIG. 2.sub.-- A.
FIGS. 2AA-2AC illustrate the starting point for the conventional process for fabricating a thin LC transducer pixel cell. Starting structure 200 is created by forming an upper intermetal dielectric layer 212 over a lower interconnect metallization layer 214. A central portion of upper intermetal dielectric layer 212 is then etched to form via 216. A liner (not shown) typically composed of a Ti/TiN layer combination, is then formed on the walls of via 216, and via 216 is filled with metal (typically CVD Tungsten). Excess metal is then removed from the surface of upper dielectric layer 212, typically by a combination of etching-and chemical-mechanical polishing (CMP).
FIGS. 2BA-2BC illustrate formation of the pixel electrode in accordance with the conventional process. Pixel electrode layer 206 is formed over the entire surface of the pixel cell. Pixel electrode layer 206 actually consists of three separate layers. Pixel electrode layer 206 is formed by the deposition of an electrode liner layer 210, typically a Ti film, to promote adhesion of the metal electrode to the underlying intermetal dielectric. Next, metal electrode layer 208, typically an Al/Cu mixture, is formed over electrode liner layer 210. A passivation layer 220 is then formed on top of metal layer 208.
FIGS. 2CA-2CC illustrate delineation of pixel electrode layer 206 into discrete electrodes of individual pixel cells in accordance with the conventional process. During this step, a photoresist mask is patterned over pixel electrode layer 206, and then unmasked regions of the pixel electrode layer 206 are etched to form a plurality of intersecting trenches 218. Intersecting trenches 218 in turn define a plurality of pixel cell electrodes 230.
FIGS. 2DA-2DC illustrate the first step of forming support pillars in accordance with the conventional process. In this first step, a thick dielectric layer 232 (typically SiO.sub.2 or Si.sub.3 N.sub.4) is deposited at high temperatures (300 to 400.degree. C.) over the entire surface of the pixel cell, including trenches 218. Because trenches 218 are relatively narrow, dielectric material is typically deposited at a faster rate at the corners of trenches 218. Uneven deposition of dielectric material within trenches 218 may be further exacerbated when the trench exhibits a re-entrant side wall profile. The uneven deposition rate of dielectric material ultimately gives rise to formation of keyhole voids 234 within the trenches.
FIGS. 2EA-2EC illustrate the second step of forming support pillars in accordance with the conventional process. In this second step of pillar formation, a pattern of-photoresist is deposited on top of thick dielectric layer 232. Unmasked portions of thick dielectric layer 232, typically located at the corners of the pixel cells 230, are then etched to leave support pillars 205. Because of the necessity of etching through all of thick dielectric layer 232, passivation film 220 and some portion of metal layer 208 are also typically exposed to etchant during this step.
Fabrication of the thin LC transducer pixel cell is completed by forming an alignment surface (not shown) for the LC material positioned on top of the pixel electrode. Forming this alignment surface is a two step process. First, a dielectric film (typically polyamide) is deposited on top of the pixel electrodes. Second, the dielectric film is scored by a rubbing wheel, which traverses the surface of the-pixel cell and gouges the alignment surface in a uniform-direction. Liquid crystal material is then placed within the cell, and a top glass plate is secured to the tops of the support pillars.
The conventional fabrication process described above is adequate to produce functional thin LC transducer pixel cells. However, the conventional process flow suffers from a number of serious disadvantages.
One problem is that the step of depositing thick dielectric layer 232 on top of pixel electrode layer 206 as shown in FIGS. 2DA-2DC produces hillocks in the metal electrode layer 208. These hillocks are due to the forces exerted by the formation of the thick dielectric layer from which the support pillars are formed. Specifically, there is a tensile stress present in metal electrode layer 208. Exposure of metal layer 208 to the heat of deposition of the dielectric (typically about 300-400.degree. C.) induces a relaxation of the metal surface and creates hillocks. These hillocks render the surface of the pixel electrode uneven, degrading the reflectance of the pixel cell.
Therefore, there is a need in the art for a process of forming a thin LC transducer pixel cell having support pillars that does not require the deposition of a thick dielectric layer directly on top of the pixel electrode.
A second problem associated with the conventional method is that the step of etching the thick dielectric layer 232 from the surface of the pixel electrode layer 206 as shown in FIGS. 2EA-2EC also degrades reflectance of the pixel electrode. When metal electrode layer 208 is freshly deposited as shown in FIGS. 2BA-2BC, metal layer 208 is extremely smooth and exhibits high reflectance. However, etching of the dielectric layer to form the support pillars as shown in FIGS. 2DA-2DC can roughen and/or oxidize the surface of the metal layer, lowering its reflectance.
Therefore, there is a need in the art for a process of forming a thin LC transducer pixel cell having support pillars that does not etch or roughen the surface of the pixel electrode.
A third problem associated with the conventional method is the formation of keyhole voids 234 as shown in FIGS. 2DB-2DC. Because of the narrow width of trenches 218, thick dielectric layer 232 is typically deposited at a faster rate along the corners of trenches 218. This differential rate of deposition ultimately promotes formation of keyhole voids 234 within support pillars 205. Keyhole voids 234 can weaken the structural integrity of the support pillars 205. Weakening of the support pillars 205 by keyhole voids 234 can be especially problematic during subsequent formation of the LC alignment surface, as the rubbing wheel utilized to score the alignment surface can contact the support pillars, subjecting them to high stress.
Therefore, there is a need in the art for a process for fabricating a thin LC transducer pixel cell that eliminates the formation of keyholes inside of the support pillars as described above.
A fourth problem associated with the conventional method is the creation of significant topography on the pixel surface that can result in optical degradation.
Liquid crystal material overlying the pixel electrode has the propensity to align and/or tilt to conform to the shape of any topology present on the surface of the pixel cell. LC alignment is a critical system attribute. The alignment of the twisted nematic LC dictates which polarization of incident light will pass through the LC's volume. In the context of a complete system, this alignment of the LC material defines either the black or white extreme of the-light valve's gray scale. As a result, non-uniformity in alignment due to the presence of surface topology will translate into a poorly constructed display.
In FIGS. 2CA-2CC, the pixel electrode layer is etched to create a plurality of discrete pixel electrodes electronically isolated from one another by a series of intersecting trenches having side walls and a trench bottom. These trench features can cause the overlying LC material to align with the trench. This misalignment can cause unwanted distortion of the image formed using the light valve.
In addition, the surface topology associated with the isolation trenches of FIGS. 2CA-2CC can also interact directly with incident light, causing reflection that is not harmonious with that of the main body of the pixel electrode. The interaction of light with the pixel topography is due to the isolation edges of the pixel. For example, in the pixel array shown in FIGS. 1A-1C, light will scatter from the sidewalls and bottom of the trenches present at the pixel edges. This unwanted scattering reduces the specular reflection of the pixel, and increases optical cross-talk between pixels.
Therefore, there is a need in the art for a process of forming a thin LC transducer pixel cell that creates a minimum of surface topology on the pixel cell surface while maintaining electrical isolation between pixel electrodes.